This application is based on Japanese Patent Application Hei 11-331500 filed on Nov. 22, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a twist-nematic (TN) liquid crystal display, and in particular, to a twisted-nematic liquid crystal display of which display quality is improved as compared with the prior art.
In a nematic state of liquid crystal, liquid crystal molecules linearly elongated in a rod-like shape have optical axes aligned in one direction or orientation.
In a twist-nematic state of liquid crystal, all liquid crystal molecules sandwiched between two substrate surfaces are aligned in parallel with the substrate surfaces, but their orientation is 90xc2x0 twisted between both substrate surfaces. Therefore, the orientation of alignment of liquid crystal molecules are continuously changed by a total of 90xc2x0 between the substrate surfaces.
2. Description of the Related Art
FIG. 5(A) and FIG. 5(B) show schematic configurations of a twist-nematic liquid crystal display.
As can be seen in FIG. 5(A), a twist-nematic liquid crystal display A includes two glass substrate 101 and 103 which are arranged to each other and apart from each other by predetermined distance d and which are opposing to each other, two transparent electrodes 105 and 107 formed on inner surfaces respective of the glass substrates 101 and 103, nematic liquid crystal EM (to be represented as Np liquid crystal herebelow) which is sandwiched between two glass substrates and which is positive in dielectric anisotropy, and a pair of polarizing plates 111 and 113 disposed outside the glass substrates 101 and 103, respectively.
Two glass substrates 101 and 103, tow transparent electrodes 105 and 107, and molecules of the nematic liquid crystal EM form pixels 115. The transparent electrode 105 is a pixel electrode of segment type and the transparent electrode 107 is a shared or common electrode. The system further includes a voltage source 117 to apply a desired voltage between the transparent electrodes 105 and 107. The system includes a plurality of transparent pixel electrodes 105. FIG. 5(a) shows a state in which no voltage is applied between the transparent electrodes 105 and 107.
A large number of molecules of the nematic liquid crystal EM in the cell 115 has a twist pitch which is sufficiently larger than a wavelength of visible light. When linearly polarized light is vertically incident to the glass substrate 103, the light is polarized 90xc2x0 (90xc2x0 polarization) along the twisted state of the molecules of the nematic liquid crystal EM while passing through the cell 115. In the cell 11, when two polarizing plates 111 and 113 have polarizing axes in a parallel Nicols state, namely, when the polarizing axes are parallel to each other, namely, the light is interrupted.
FIG. 5(B) shows a state in which a voltage is applied between the transparent electrodes 105 and 107.
When a predetermined voltage applied to the pixel 115 from the voltage source 117 and the voltage exceeds a threshold voltage Vth, long axes of liquid crystal molecules EM start changing their direction toward a direction of an electric field associated with the voltage.
When the voltage is about twice the threshold voltage Vth, the long axes of liquid crystal molecules in other than the neighborhood of surfaces of the transparent electrodes 105 and 107 are uniformly re-aligned to be parallel to the direction of the electric field and the polarization of 90xc2x0 is lost. In this state, contrary to the state in which no voltage is applied between the transparent electrodes 105 and 107, light is allowed to pass therethrough in the parallel Nicols state.
In the twist-nematic liquid crystal display A, when the polarizing plates 111 and 113 are arranged in the parallel Nicols state, the liquid crystal display can be operated in a normally black mode.
The liquid crystal molecules EM are sandwiched between two glass substrates 101 and 103 of the display A. To increase contrast between white and black in displayed images, thickness d of the cell of liquid-crystal molecules EM is determined as follows.
When 0 voltage is applied between the transparent electrodes 105 and 107, the pixels of the liquid-crystal display has transimittivity expressed as
T=sin2{xcfx80(1+u2)0.5/2}/(1+u2)xe2x80x83xe2x80x83(3)
where, u=2xcex94nd/xcex and xcex94n=(npxe2x88x92n1).
In expression (3), xcex is a wavelength of the incident light and np is a refractive index of liquid crystal molecules in an axial direction parallel to the long axis of the liquid crystal molecules, and Nl is a refractive index of the liquid crystal molecules in an axial direction vertical to the long axis of the liquid crystal molecules. By determining the value of d for which the transmittivity T of the pixels takes a minimum (minimal) value in expression (3), images can be displayed with a high contrast ratio.
The function of expression (3) is a periodic function. Therefore, the transmittivity takes a plurality of minimal values. Namely, a plurality of values of d exist for the minimal values. Usually, a first or second minimal value of d at a lower order is used in ordinary cases.
In the liquid crystal displays using twist-nematic liquid crystal, images are displayed in primarily two ways, namely, display of segment type and display of matrix type.
The liquid crystal display of the segment type includes a relatively small number of segment-type pixel electrodes and is suitable when a limited number of symbols, numerals, and the like are repetitiously displayed. The liquid crystal displays of segment type are broadly used, for example, for desktop computers (calculators), watches, measuring instruments, computer game machines, and devices to display bar graphs.
In the liquid crystal display of matrix type, a large number of pixel electrodes are arranged, for example, in a contour of a simple matrix. Liquid crystal displays of matrix type are suitably used to display complex images and the like and are employed, for example, for personal computers and television sets.
The liquid-crystal displays are operated by static or multiplex driving.
The static driving is mainly employed when the display includes a small number of pixel electrodes. Each of the pixel electrode of the liquid crystal display receives a voltage individually from a driving circuit (driver). During a display period, the driver continuously applies a display voltage Vs between segment-type pixel electrodes of pixels to be displayed and the common electrode. All segment-type pixel electrodes can be simultaneously driven.
In the liquid-crystal display operated by static driving, 0 volt is applied to segment-type pixel electrodes for a non-display section (not selected for the display operation). Namely, this voltage is the same as that applied to a base section in which no segment-type electrode exists and only a common electrode exsist. The transmittivity and hue in the non-display section are almost the same as those in the base section. Namely, the non-display section and the base section can be readily discriminated from a display section.
The multiplex driving is also called time-division driving or dynamic driving and is used, for example, in a liquid crystal display of segment type including a relatively large number of segment-type pixel electrodes to be displayed. The multiplex driving is also applied to a liquid crystal display of matrix type having a large number of pixel electrodes.
FIGS, 6 and 7A-7C show an example of the multiplex driving in a liquid crystal display.
FIG. 6 shows wiring of a liquid crystal display B in which numerals are displayed at 12 places or positions. Each numeral is displayed with seven segments by multiplex driving. FIG. 6 shows an example in which only three places of numerals are displayed.
For each place, a common electrode 105 is subdivided into 12 sections for place electrodes Xi, i.e., X1 to X12. All segment-type pixel electrodes 107 are classified into seven groups Yi, i.e., Y1 to Y7. For each group, the electrodes 107 belonging thereto are connected to each other.
At each place, segment-type pixel electrodes (display electrodes) Yi to be displayed are selectively driven at timing synchronized with timing at which the place electrodes Xi are sequentially driven by time-division driving with a duty factor of {fraction (1/12)}.
The configuration of the liquid crystal display requires 19 driver elements and 19 lead terminals. The number of driver elements and that of lead terminals are remarkably reduced when compared with a case in which the static driving is used.
However, since a large number of pixel electrodes 107 are commonly connected, various divided voltages are applied to the pixel electrodes (non-display electrodes) 107 other than those driven for the display operation. This decreases contrast in displayed images and hence leads to a phenomenon of so-called xe2x80x9ccrosstalkxe2x80x9d.
The crosstalk occurs because the liquid crystal equally responds to positive and negative voltages and has a slow electrochemical response (voltage-display contrast) characteristic.
To minimize influence of the crosstalk, a voltage averaging method is employed when the liquid crystal display is driven by multiplex driving.
FIGS. 7A-7C show signal timing charts of driving pulse signals in a liquid crystal display operated by n-division driving in a voltage averaging method.
FIG. 7(A) is a signal timing chart showing operation to apply signal voltages V(X1) to V(Xn) to the common electrodes, i.e., the place electrodes X1 to Xn. Assume that one scan period is expressed as TF. A pulse signal of V0 is applied to one place electrode Xi for a period of TF/n.
In one scan period thereafter, a pulse of xe2x88x92V0 is applied thereto.
As can be seen from FIG. 7(B), throughout one scan period, a voltage of xe2x88x92V0/a and a voltage of V0/a are applied to the pixel electrode Yi in a data-on state and in a data-off state, respectively. In a subsequent period, V0/a and xe2x88x92V0/a are respectively applied thereto in the data-on and data-off states, respectively.
As shown in FIG. 7(C), when a signal voltage of xc2x1V0 is applied to the common electrode, the voltage V(Xi, Yi) applied between the pixel electrode and the common electrode is xc2x1(1+1/a)V0 with respect to each display electrode and is xc2x1(1xe2x88x921/a)V0 with respect to each non-display electrode. When the signal voltage of xc2x1V0 is not applied to the common electrode, the voltage V(Xi, Yi) applied between the pixel electrode and the common electrode is xc2x1V0/a with respect to each display electrode and is xe2x88x92(xc2x1V0/a) with respect to each non-display electrode.
Assume in the voltage averaging method that the scanning pulse and the data input signal pulse are V0 and V0/a in the time-division driving and the number of scanning electrodes is n. An effective voltage Vs which is an effective voltage applied to a liquid crystal layer of a selected pixel and an effective voltage Vns which is an effective voltage applied to a liquid crystal layer of a non-selected pixel are expressed as follows.
Vs=(V0/a){(a2+2a+n)/n}0.5xe2x80x83xe2x80x83(4)
Vns=(V0/a){(a2xe2x88x922a+n)/n}0.5xe2x80x83xe2x80x83(5)
An optimal value of a in expressions (4) and (5) can be obtained when the display contrast takes a maximum value, namely, when Vs/Vns becomes maximum. The optimal value is obtained as follows.
a=n0.5xe2x80x83xe2x80x83(6)
In a normally-black liquid crystal display operated by time-division driving, contrast between pixels is defined as Ts/Tns, where Ts is transmittivity of a pixel to which a selection signal is applied and Tns is transmittivity of the pixel to which a non-selection signal is applied.
When the twist-nematic liquid crystal display is operated by time-division driving, a predetermined voltage is applied between the common electrode and the pixel electrodes also in the non-selected state (for the pixels not to be displayed).
Therefore, the transmittivity of the base section differs from that of the section of non-selected pixels (pixels not to be displayed). Namely, these sections vary also in hue from each other.
Since the non-display section and the base section differ in transmittivity and hue from each other, the non-display section cannot be clearly discriminated from the display section depending on cases.
To facilitate discrimination of these sections in the liquid crystal display operated by the time-division driving, it is desired that the transmittivity Tns of the pixel to which a non-selection signal is applied is substantially equal to transmittivity T0 of the pixel in the base section. When Ts/Tns takes a large value and Tns=T0, only a desired display section can be clearly discriminated and hence the liquid crystal display is improved in display quality.
FIG. 8 shows dependence of transmittivity T of pixels on retardation xcex94nd in a twist-nematic liquid crystal display of the normally black mode in the prior art.
In the graph of FIG. 8, small circles represent a characteristic of transmittivity T0 of pixels in a base section (a section in which the common electrode 105 is visible in FIG. 6). This graph shows transmittivity T0 up to a third minimal value thereof.
Small rectangles represent dependence of transmittivity of non-selection pixels on retardation xcex94nd when a non-selection voltage Vns is applied between the pixel electrodes (of segment type) of the pixels and the common electrode 105. As in expression (5), the non-selection voltage Vns is represented by xc2x1(V0/a){(a2xe2x88x922a+n)/n}0.5. The voltage is ordinarily set to a value for which the transmittivity takes a minimum value in the characteristic thereof with respect to the voltage.
The transmittivity T0 of pixels in the base section takes first, second, and third minimal values.
In contrast therewith, the transmittivity Tns of pixels in the non-selection state takes minimal values in a range of xcex94nd from 1.2 micrometers (xcexcm) to 1.3 xcexcm (a range between the second minimal value to the third minimal value of the transmittivity T0).
Consequently, in the twist-nematic liquid crystal display operated by time-division driving, the contrast (Ts/Tns) becomes higher when the retardation xcex94nd is set to a minimal value of the transmittivity (xcex94nd=1.2 xcexcm to 1.3 xcexcm as shown in FIG. 8) not in a state in which the applied voltage is zero but in a state in which the non-selection voltage is applied.
Also in an actual twist-nematic liquid crystal display, the contrast is increased by setting xcex94nd to a minimal characteristic value in a state in which the non-selection voltage is applied, for example, to a value in a range 1.2 xcexcm to 1.3 xcexcm.
By setting xcex94nd to a value in the vicinity of a minimal characteristic value in a state in which the non-selection voltage is applied, the value of Tns is lowered and a high contrast (Ts/Tns) is obtained in images displayed by the liquid crystal display.
However, even when the retardation xcex94nd is set to a value in the neighborhood of a minimal value of the transmittivity Tns, T0 considerably differs from Tns.
In a full-dot-matrix liquid crystal display in which pixel electrodes occupy almost the entire display screen area, the base section has quite a small area in many cases. Even when T0 considerably differs from Tns, there does not arise any particular problem which hinders easy discrimination between the non-display section and the base section.
However, when the base section occupies a large area as in the liquid crystal display shown in FIG. 6, the considerable difference between T0 and Tns, leads to the so-called crosstalk in which not only the section of selected pixels but also the section of pixels not selected are discriminated from the base section. This reduces clarity of images displayed by the liquid crystal display.
FIG. 9 shows a relationship between transmittivity T of pixels and a wavelength of incident light in a liquid crystal display.
In the graph, dependence of the wavelength is compared between the transmittivity T0 and the transmittivity Tns of non-selection pixels.
As can be seen from the graph, the transmittivity T0 has a maximal value for wavelengths of 420 nanometers (nm) and 550 nm and the transmittivity Tns of non-selection pixels has a maximal value for a wavelength of 390 nm. T0 differs from Tns not only in the absolute value but also in the wavelength dependence.
Therefore, the hue remarkably varies in images displayed by the liquid crystal display and the display quality is deteriorated.
The display quality is more reduced when the displayed image is watched from another angle of view.
The graph of FIG. 10 shows wavelength dependence of the transmittivity T when the liquid crystal display is viewed from an inclined direction. In the graph, the wavelength dependence of the transmittivity T0 is compared with that of the transmittivity Tns.
The liquid crystal display is viewed from position at a side thereof when a voltage is applied to the associated electrodes. Specifically, assume that the aligning direction (a direction of long axes) of liquid crystal molecules in a central section of a liquid crystal cell is the direction of 6 o""clock on a dial of a watch. The liquid crystal display is viewed from the direction of 9 o""clock on the dial. Namely, the viewing direction is 40xc2x0 inclined from a normal of the liquid crystal cell.
The transmittivity T0 has a minimal value for a wavelength of 410 nm and a maximal value for a wavelength of 500 nm. The transmittivity Tns of non-selection pixels has a minimal value for a wavelength of about 390 nm.
Comparison of the spectral characteristic between FIG. 9 and FIG. 10 implies remarkable discrepancy of the absolute value and the wavelength dependence between the transmittivity T0 and the transmittivity Tns.
In a wavelength range from 500 nm to 780 nm, the transmittivity Tns of pixels viewed from the inclined direction of FIG. 10 is disadvantageously higher than the transmittivity T0 of pixels viewed from the same direction.
The display characteristic is much more deteriorated when the liquid crystal display is viewed from the incline direction.
It is therefore an object of the present invention to provide a liquid crystal display in which even when the display is operated by time-division driving, there is obtain a satisfactory display characteristic of high contrast and reduced difference in the transmittivity T and the wavelength dependence (hue) between the base section and the section of non-selection pixels.
Another object of the present invention is to provide a liquid crystal display in which the satisfactory display characteristic can be obtained in a wide viewing angle.
To achieve the objects according to the present invention, there is provided a twist-nematic liquid crystal display including a first substrate and a second substrate disposed to oppose to said first substrate with distance d1 therebetween, either one of said first and second substrates being a transparent substrate; a plurality of pixel electrodes formed on either one of a surface of said first substrate and a surface of said second substrate, said surfaces opposing to each other; a common electrode formed on other one of said surfaces respectively of said first and second substrates; voltage applying means for applying a voltage between said pixel electrodes and said common electrode; twist-nematic liquid crystal sandwiched between said first and second substrates, molecules of said liquid crystal respectively having long axes continuously twisted between said first and second substrates. When said twist-nematic liquid crystal has a birefringence index (retardation) of xcex94n, expression xcex94nxc3x97d1 greater than 2 xcexcm is satisfied, where d, is expressed in micrometers.
According to the present invention, there is provided a twist-nematic liquid crystal display including a first substrate and a second substrate disposed to oppose to said first substrate with distance d2 therebetween, either one of said first and second substrates being a transparent substrate; a plurality of pixel electrodes formed on either one of a surface of said first substrate and a surface of said second substrate, said surfaces opposing to each other; a common electrode formed on other one of said surfaces respectively of said first and second substrates; voltage applying means for applying a voltage between said pixel electrodes and said common electrode; twist-nematic liquid crystal sandwiched between said first and second substrates, molecules of said liquid crystal respectively having long axes continuously twisted between said first and second substrates. When said twist-nematic liquid crystal has a birefringence index (retardation) of xcex94n, expression xcex94nxc3x97d2 greater than 2.2 xcexcm is satisfied, where d1 is expressed in micrometers.
According to the present invention, there is provided a twist-nematic liquid crystal display including a first substrate and a second substrate disposed to oppose to said first substrate with a distance therebetween, either one of said first and second substrates being a transparent substrate; a plurality of pixel electrodes formed on either one of a surface of said first substrate and a surface of second substrate, said surfaces opposing to each other; a common electrode formed on other one of said surfaces respectively of said first and second substrates; voltage applying means for applying a voltage between said pixel electrodes and said common electrode; twist-nematic liquid crystal sandwiched between said first and second substrates, molecules of said liquid crystal respectively having long axes continuously twisted between said first and second substrates; and a first area in which said pixel electrodes are formed and a second area in which only said common electrode is formed, said second area being in a periphery of said first area. Contrast obtained according to transmittivity of light through said first area and transmittivity of light through said second area is used to display an image. Said voltage applying means applies a selection voltage or a non-selection voltage to said pixel electrodes. The selection voltage and the non-selection voltage are set to values, the values increasing discrepancy between transmittivity of light through said first area when the selection voltage is applied to said pixel electrodes by said voltage applying means and transmittivity of light through said first area when the non-selection voltage is applied to said pixel electrodes by said voltage applying means, the values decreasing discrepancy between transmittivity of light through said first area when the non-selection voltage is applied to said pixel electrodes and transmittivity of light through said second area.
In a liquid crystal display according to the present invention, the contrast ratio takes a value which can be used without causing any problem in practices. The transmittivity of the base section is almost the same as that of the section of non-selected pixels, and substantially a completely black color can be displayed in both sections.